1. Field of the Invention
The present invention relates to an image-sensing semiconductor device and to an image-sensing device.
2. Description of the Prior Art
An image-sensing device used to read an original image has a plurality of image-sensing semiconductor devices (semiconductor chips) arranged therein, of which each has a plurality of photodiodes (photoelectric conversion devices) arranged in a row. As shown in FIG. 18, each semiconductor chip has a photodiodes D1, D2, . . . , Dn, amplifying transistors A1, A2, . . . , An, constant-current sources I1, I2, . . . , In, and switching transistors C1, C2, . . . , Cn. As pulses are fed in sequentially at terminals O1, O2, . . . , On, the transistors C1, C2, . . . , Cn are turned on sequentially, so that the photoelectric conversion signals from the photodiodes D1, D2, . . . , Dn are delivered to an output terminal 100.
However, the photodiodes have variations in characteristics inevitable in their manufacture from one chip to another, and therefore the photoelectric conversion signals output therefrom include errors due to variations from one chip to another. On the other hand, as shown in FIG. 20, the switching transistors C1, C2, . . . , Cn each have parasitic capacitances 101 and 102 between their gate G and source S and between their gate G and drain D. Accordingly, a switching voltage fed in via the terminal O and applied to the gate G affects, through those capacitances 101 and 102, the voltages at the source S and drain D.
For example, the switching transistor C2 of the second pixel is turned on at a time point P2 in FIG. 19 and, at this time, as the switching voltage drops, the voltage at its drain (and thus the output voltage of the photodiode D2) drops. However, at this time point P2, the switching transistor C1 of the first pixel is turned off, and therefore the rising voltage at its gate appears at its drain and thereby raises the voltage on the output line 105.
This means that the effect of the dropping voltage of the switching signal for the switching transistor C2 is canceled by the effect of the rising voltage of the switching signal for the switching transistor C1. Similarly, also with the third and following pixels, the effect of the switching voltage therefor is canceled by the switching voltage for the previous stage, and thus does not appear at the output terminal 100.
However, with the first pixel, at the time point P1 when the switching signal for the switching transistor C1 drops, no voltage is present that acts to cancel the effect of this dropping voltage. As a result, with the first pixel, it is conventionally inevitable that such a dropping voltage is mixed with and output together with the desired signal.
An object of the present invention is to provide an image-sensing semiconductor device and an image-sensing device employing such an image-sensing semiconductor device that permit individual chips to output photoelectric conversion signals free from errors due to variations in characteristics among the chips.
Another object of the present invention is to provide an image-sensing semiconductor device and an image-sensing device employing such an image-sensing semiconductor device that eliminate the effect of a switching signal on the photoelectric conversion signal from the first pixel within a chip.
To achieve the above objects, according to one aspect of the present invention, an image-sensing semiconductor device is provided with: a plurality of image-sensing photodiodes; a dummy photodiode; first bias means for feeding a bias to the dummy photodiode repeatedly with a predetermined period; second bias means for feeding a bias to the plurality of photodiodes sequentially with a predetermined cycle; means for feeding the output signals of the plurality of photodiodes sequentially to a first input terminal of a differential amplifier; means for feeding the output signal of the dummy photodiode to a second input terminal of the differential amplifier; and output means for feeding the output of the differential amplifier to an output terminal.
This image-sensing semiconductor device may be further provided with means for eliminating a direct-current component from the output of the differential amplifier and means for superimposing a new direct-current voltage on the output of the differential amplifier after elimination of the original direct-current component therefrom. This helps avoid the effects of offsetting resulting from amplification or other.
Alternatively, the image-sensing semiconductor device may be further provided with an output switch through which the output from the differential amplifier passes before reaching the output terminal, output control means for keeping the output switch in a conducting state until the output signals from all of the photodiodes have passed therethrough, and means for generating, in accordance with a clock, pulses to be used to feed the output signals of the photodiodes to the differential amplifier. In this case, the output control means produces a passage control signal from the clock and has delay means for extending the passage control signal. This helps avoid malfunctioning resulting from a delay along the signal path (i.e. malfunctioning in which the switch is closed before the photoelectric conversion signal is output from the last photodiode).
By forming this image-sensing semiconductor device on a single chip, it is possible to form an image-sensing device having a plurality of semiconductor chips arranged so as to form a line, with the semiconductor chips each having a plurality of image-sensing photoelectric conversion devices arranged in a row. In this case, a dummy photoelectric conversion device is provided in each of the semiconductor chips, and, from one semiconductor chip after another, a photoelectric conversion signal is output that represents the differences between the output of the dummy photoelectric conversion device and the individual outputs of the image-sensing photoelectric conversion devices.
According to another aspect of the present invention, an image-sensing semiconductor device is provided with: a plurality of image-sensing photodiodes; a dummy photodiode; first bias means for feeding a bias to the dummy photodiode repeatedly with a predetermined period; second bias means for feeding a bias to the plurality of image-sensing photodiodes sequentially with a predetermined cycle; a plurality of switching transistors for feeding the output signals of the plurality of image-sensing photodiodes sequentially to a first input terminal of a differential amplifier; means for feeding the output signal of the dummy photodiode to a second input terminal of the differential amplifier; and a dummy switching transistor having an input terminal connected so as to receive the output signal of the dummy photodiode, having an output electrode connected to the first input terminal of the differential amplifier, and controlled by a switching voltage applied to a control electrode thereof in such a way as to be turned off substantially simultaneously when the switching transistor corresponding to the first of the image-sensing photodiodes is turned on.